Thermal conductor assembly

ABSTRACT

A compressible thermally conductive member comprises a polymer field with thermally conducting magnetically aligned particles comprising a base portion and a multiplicity of protrusions extending from at least one surface of the base portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an assembly having means for conducting heataway from a heat producing member.

2. Related Art

In the electronic industry, especially where high power and/or a highdensity of devices are employed there is often a need to provide meansfor dissipating the heat generated by or at such devices. Without suchheat dissipation the life of such devices may be adversely andcatastrophically affected. In the past, heat dissipation wasaccomplished by means of blowing air over the device to be cooled and/orcontacting the device with a heat sink which may be provided withcooling fins for increasing the surface area and hence the efficiency ofheat dissipation. Generally, such heat sinks have been rigid metallicbodies or rigid heat conductive ceramics. In addition, elastomers filledwith heat conductive particles have been employed to conduct heat awayfrom certain devices. However, where the device to be cooled is mountedin an enclosure or housing or where there is a variation in spacingbetween such device and the enclosure or heat sink, it is oftendifficult to provide good thermal conduction of heat from the device tothe the sink or the housing to dissipate the heat from the device andprevent excessive heat buildup within the housing. One problem has beenthe difficulty in making good contact between the device and the meansemployed for dissipating the heat without undue pressure on the device.

SUMMARY OF THE INVENTION

We have now developed a highly compressible, thermally conductive memberwhich, when positioned under compression adjacent a heat source canefficiently transfer heat through the member away from the heat source.

The compressible thermally conductive member comprises a polymer filledwith thermally conducting, magnetically aligned particles such that, atleast when under compression, a thermal path exists across the thicknessof the member.

The magnetic alignment of the particles form a multiplicity of columnsextending across the thickness of the member and wherein, on at leastone surface, the columns extend from the surface outwardly to form amultiplicity of finger-like protrusions.

We have surprisingly discovered that such magnetically aligned thermallyconductive particles in a polymer matrix result in significantly greaterthermal conductivity across the thickness of the polymer (typically, atleast twice the conductivity for the volume loading generally employed)or compared with structures of the same particles and particle loading,but not magnetically aligned,

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a representation of an electronic assembly which includesnovel means for dissipating heat from the assembly.

FIG. 2 is a schematic cross-sectional representation of a compressiblethermal conductor useful in the present invention.

FIGS. 3 & 4 are schematic cross-sectional representations of otherembodiments of a compressible thermally conductive member.

DETAILED DESCRIPTION

In accordance with FIG. 1, there is shown an electronic assembly 1 whichcomprises a plurality of heat-producing electronic devices 2 stackedadjacent and spaced from one another, a compressible resilient thermallyconductive sheet 3 abutting portions of the electronic devices 2 and ametal enclosure 4 surrounding the device and abutting the thermallyconductive sheet 3. At least a part of the enclosure 4 may constitute adoor of the enclosure which presses against and compresses a majorsurface of the sheet 3 to allow the heat conducted from the device 2 topass through the sheet 3 to the enclosure 4 thence and be dissipatedinto the atmosphere. The assembly may further include a fan (not shown)to aid in cooling. In place of the enclosure, it is undrestood that anytype of heat sink or heat dissipating means can be employed. Generally,in an assembly as shown in FIG. 1 there is a large variation in spacingbetween the enclosure surface and the various devices therein. Thisgives rise to the end for a highly compressible yet thermally conductivemember, such as the thermally conductive sheet described herein, toeffectively cool all of the devices.

Referring to FIG. 2 there is shown a thermally conductive sheet 21comprising a compressible resilient compliant polymer matrix 22 havingimbedded therein an array of thermally conducting magnetic particles 23.The particles 23, as shown, are aligned in spaced columns 24. Theparticles 23 within a column 24 abut one another or are otherwisesufficiently close, that at least under compression, will conduct heatalong the column length from one major surface of the sheet to theother. One major surface 25 of the sheet 21 is essentially flat. Theother major surface 26 of the sheet 21 is provided with a multiplicityof finger-like protrusions 27 comprising columns 24 of thermallyconductive particles 23 embedded in the polymer matrix 22. Theseprotrusions may extend to such a length that at least upon beingdeformed under an applied pressure, adjacent protrusions overlap oneanother or alternatively may be of shorter lengths. The overlapping ofthe protrusions allows for heat dissipation laterally. across the sheet,as well as through the sheet thickness while still affording highcompressibility of the sheet with minimal applied pressure. Generally,the greater the height of the protrusion (prior to sagging of theprotrusion) the more compressible the member with a given applied force.Optionally, the base of the sheet 21 may have embedded therein or on theflat surface 24 a thermally conducting screen 28. The screen ispreferably made of a magnetic metal and acts not only to enhance lateralthermal conductivity, but aids in the formation of a uniform array ofcolumns of magnetic particles in the sheet 21.

The polymer may be any polymer which preferably is compliant andresilient when loaded with the thermally conductive particles. Examplesof suitable polymers are silicon elastomers, and flexible epoxies andpolyurethanes. A heat curable silicone elastomer is generally thepreferred polymeric material. Further, the particle loaded polymer maybe formed by any of the commonly known techniques including casing,drawing and extrusion and may be in the form of a foam or solid.

Particles which are suitable for use, while preferably being sphericalare not limited by their shape and may even be in the form of flakes.Suitable particulate materials which are thermally conducting andmagnetic are metals such as nickel, cobalt, and iron and their magneticalloys. Other suitable magnetic, thermally conducting particles includemagnetic thermally conducting ceramics, oxides and intermetalliccompounds as are known in the art. Preferred particles are oftencomposite materials, for example, as nickel having a coating of copperthereover to increase its thermal conductivity and a second coating of arelatively inert metal such as silver to protect the copper fromoxidation. Alternatively, the composite particle may have a non-magneticcore with a magnetic coating. Also, in addition to the magneticparticles, the thermally conductive member may incorporate non-magneticthermally conductive particles such as non-magnetic metals, e.g. copperor ceramics, e.g. alumina and beryllia.

In order to achieve the preferred columnar structure, the particles arealigned by applying a magnetic field to the particles in the polymerduring curing of the polymer.

The thickness of the sheet is not critical and is dependent upon theparticular use to which it is to be put and the maximum and/or minimumpressure to be applied in use. Typical thicknesses for the main body ofthe sheet are from 1 to 100 mils with protrusions extending therefrom,an additional 5 to 500 mils. The length of the protrusions are at leastequal to a multiple of the particle diameter so that at least severalparticles are incorporated in the protrusion.

For certain applications, where the total area of the sheet within anenclosure is large, for example several square meters, and/or wherethere is a significant variation in surface uniformity of the device ordevices to be cooled and/or the dissipating medium or enclosure, it isextremely important that the sheet be able to be highly compressiblewith small applied pressures to insure sufficient contact area of thesheet and the device dissipating medium between which the sheet isinterposed. Generally, deformations in the order of from 10% to 50% ofthe original overall thickness of the sheet under applied pressures oftypically about 10 psi are attained. Such a sheet can be achieved, forexmaple, by employing a silicon elastomer as the polymeric matrix loadedto about 50 volume percent with essentially spherical nickel basedparticles having a particle size of from 1 to 10 mils which are alignedto form the desired columnar structure.

FIGS. 3 and 4 show alternative structure so the novel thermallyconductive sheet material. It should be noted that the sheet materialmay have other uses in addition to heat transfer, e.g. as a gasketmaterial and as an electrical conductor, such as to provide a groundplane when the thermally conductive particles also provide anelectriclly conducting path. Further, one may interpose another thermalconductor, e.g. a metal foil or sheet between the magnetically alignedthermally conductive polymer sheet and/or the device to be cooled andthe heat dissipator. This could enhance thermal conduction. Also, thepolymer sheet can be employed between heat producing devices when thereis also lateral thermal conduction toward the edges of the sheet so asto aid in heat dissipation.

The following table provides a comparison of characteristics as betweena 175 mil thermally conductive silicone elastomeric sheet filled to 30%by volume with 3 mil mean diameter gold located nickel spheres which arealigned to provide the structure having protrusions as described withreference to FIG. 2, without the optional screen and a similar materialwhich does not have the protrusions but is essentially flat on bothmajor surfaces.

                  TABLE I                                                         ______________________________________                                         Comparison between novel Thermally Conductive                                Sheet and a similar sheet without protrusions                                               Novel Sheet                                                                             Flat Sheet                                            ______________________________________                                        Displacement (at 5 psi)                                                                       85 mil      5 mil                                             (Reduction in Thickness)                                                      % Deformation (at 5 psi)                                                                      48.6%       2.9%                                              R (Z-Direction Contact                                                                        25-75 mΩ                                                                            2800-4700 mΩ                                Resistance, 25 × 50 mil                                                 Pad Area)                                                                     Thermal Conductivity                                                                           0.745      1.74                                                              watt/m. °C.                                                                        watt/m. °C.                                                (at 12 psi) (at 20 psi)                                       ______________________________________                                    

As can be seen from the Table, the structure having the protrusions issignificantly more deformable than its flat counterpart and surprisinglyexhibits a compatable thermal conductivity to its flat counterpart.

In manufacture, the height, density and shape of the thermallyconductive protrusions can be controlled by adjusting one or more of theprocess or materials parameters, e.g. the magnetic field directionand/or strength, the elastomer viscosity and the particle loading.

One method of making a highly durable, compressible, thermallyconductive sheet is to form a composite from two or three discretelayers. Such a sheet is shown in FIG. 3. The first layer 30 provides abase to support the second layer 31. The second layer 31 forms thefinger-like protrusions 32 and the optional third layer 33 covers theprotrusion 32 so as to increase durability as well as lateral heatconductivity. Layers 31 and 33 are monolithic in the final assembly withlayer 33 being on top of the protrusions. This sandwich structure may beheld in a frame running along the periphery of the composite. It shouldbe noted that the protrusions may be formed at an angle other thannormal to the base material. This would enhance ease and uniformity inthe compression of the sheet.

By way of example, all three layers are preferably made of the sameprecursor materials, e.g. nickel particles coated with about 25 micronsof copper to enhance thermal conduction and about 0.6 microns of arelatively inert metal such as silver or gold to prevent oxidation ofthe copper. Particles with a mean particle size range of about 7 to 10mils are employed. Heat curable silicone elastomers which have anelongation factor before rupture of at least 350%, a Shore A durometerof less than 50 and a tear strength of at least 85 lbs. per inch arepreferred. An example of such an elastomer is Dow Corning Silastic E.

The first and third layers are made with a high particle loading, e.g.about 50 volume percent. The first layer is drawn to a thickness oftypically 15 to 25 mils and the particles are aligned to form a columnarpattern by means of a magnetic field of about 300 oersteds for twominutes with partial heat curing at 110 degrees C. The second layer isdrawn over the partially cured first layer to a thickness of 20 mils.The second layer generally has a particle loading of between 30 to 50volume percent. This layer is drawn to form the protrusions using amagnetic field of about 1100 oersteds with the protrusion height beingcontrolled by the size of the magnetic gap. This composite is cured in amagnetic field at about 1100 degrees C. for about 15 minutes. The thirdlayer is made in the same manner as the first layer and applied over thecomposite of the first two layers. All sections of the composite arethen additionally cured for about ten hours, preferably in an inertatmosphere, e.g. nitrogen or argon, at about 140 degrees C. It should benoted that, in use, two or more sheets with protrusions on one surfacecan be coupled, or a sheet can be folded over itself, such that theouter surfaces are both flat, both have the protrusions or one is flatand one has the protrusions.

What is claimed is:
 1. An assembly comprising;(a) a heat producingmember, (b) a compressible compliant sheet comprising a base portionhaving a multiplicity of spaced columns of magnetically aligned,thermally conductive magnetic particles across the thickness of saidsheet and a multiplicity of finger-like protrusions containing saidparticles extending outwardly from at least one surface of said baseportion, said particles being in a polymer matrix each of saidprotrusions extending from said surface a distance significantly largerthan the width or thickness of such protrusion, said sheet lyingadjacent said member so as to allow the transfer of heat from saidmember through said sheet, and (c) means for compressing said sheet. 2.The assembly recited in claim 1 wherein said polymer is an elastomer andwherein said magnetically aligned particles comprise magnetic materialselected from the group consisting of a metal, a metal alloy, an oxideand an inter-metallic compound.
 3. The assembly recited in claim 2wherein said elastomer is a heatcurable silicone and wherein saidparticles comprise a member of the group selected from nickel and anickel alloy.
 4. The assembly recited in claim 1 wherein said particlesare a composite having a magnetic core, a highly thermally conductivematerial around said core and an oxidation resistant, thermallyconductive layer around the thermally conductive metal.
 5. The assemblyrecited in claim 4 wherein the magnetic core of said particles comprisesnickel, the thermally conductive material arround said core comprisescopper and said oxidation resistant layer comprises silver.
 6. Theassembly recited in claim 1 wherein the particle size of saidmagnetically aligned particles are from 1 to 10 mils.
 7. The assemblyrecited in claim 1 including means associated with said sheet forproviding lateral conduction of said heat.
 8. The assembly recited inclaim 7 wherein said protrusions overlap at least when said sheet iscompressed.
 9. The assembly recited in claim 7 including a thermallyconductive layer abutting said sheet.
 10. The assembly recited in claim9 wherein said thermally conducting layer abuts said protrusions andcomprises thermally conductive particles dispersed in a polymericmatrix.
 11. The assembly recited in claim 1 wherein the sheet isdeformable by at least 10% of its initial thickness under a pressure of10 psi.
 12. The assembly recited in claim 11 wherein said base portionhas a thickness of from 1 to 100 mils and said protrusions extend from 5to 500 mils therefrom.
 13. The sheet recited in claim 1 wherein saidsheet is compressible from 10 to 50% of its original thickness under anapplied pressure of 10 psi.
 14. An assembly comprising(a) a plurality ofspaced heat-producing members mounted in an enclosure (b) compressiblemeans thermally coupled to said heat-producing members for conductingheat away from said plurality of heat-producing members, and (c) meansfor dissipating said heat from said enclosure which is thermally coupledto said compressible means wherein said compressible means comprises oneor more sheets of a compliant elastomeric polymer having dispersedtherein thermally conductive, magnetic particles which have beenmagnetically aligned so as to form spaced columns of said particlesacross the thickness of said sheet including a base portion and amultiplicity of protrusions extending from said base portion.
 15. Theassembly recitded in claim 14 wherein said enclosure includes a door,said heat-dissipating means is the door of said enclosure, and saiddoor, when closed, compresses said compressible means against saidheat-producing members.
 16. A compressible thermally conductive sheetcomprising a base portion and finger-like protrusions extendingoutwardly from at least one major surface of said base portion, saidsheet comprising a compliant polymer filled with thermally conductive,magnetically aligned magnetic particles forming a multiplity of spacedcolumns in said polymer.
 17. The thermally conductive sheet recited inclaim 16 wherein said protrusions are of such a length as to overlap oneanother at least when said sheet is compressed.
 18. The thermallyconductive sheet recited in claim 16 wherein said particles arecomposites of two or more layers.
 19. The thermally conductive sheetrecited in claim 18 wherein said particles comprise a magnetic core andmetalic layer over said core, said metallic layer having a higherthermal conductivity than said core.
 20. The thermally conductive sheetrecited in claim 19 wherein said particles further comprise an outerlayer of a highly thermally conductive, oxidation resistant materialover said metallic layer.
 21. The thermally conductive sheet recited inclaim 20 wherein said core is selected from the group consisting of Ni,Fe, Co, alloys of at least one of Ni, Fe and CO and oxides, the metalliclayer is selected from the group consisting of Cu and Cu alloys and saidouter layer is selected from the group consisting of Ag, a noble metaland alloys thereof.
 22. The sheet recited in claim 16 wherein saidpolymer is a polysiloxane.
 23. The sheet recited in claim 16 includingthermally conductive layer over and in thermal contact with saidprotrusions.
 24. The sheet recited in claim 16 including thermallyconductive nonmagnetic particles dispersed in said base portion.
 25. Thesheet recited in claim 16 wherein said base portion is from 1 to 100mils thick and said protrusions are from 5 to 500 mils in length, saidprotrusions containing a plurality of particles.
 26. A compressiblethermally conductive sheet comprising a base portion and finger-likeprotrusions extending from at least one major surface of the baseportion, said sheet comprising a compliant polymer filled with thermallyconductive particles forming a multiplicity of spaced columns in saidpolymer, said protrusions being of such a length as to overlap oneanother at least when said sheet is compressed.
 27. A compressiblethermally conductive sheet comprising a base portion and finger-likeprotrusions extending outwardly from at least one major surface of saidbase portion, said sheet comprising a compliant polymer filled withthermally conductive particles forming a multiplicity of spaced columnsin said polymer, said particles being composites of two or more layers.